Demoting an Employee after an off-work accident - how should I document this to protect the company?

We have an employee who was in a car accident during his personal time and sustained a head injury that has affected his short term memory where he forgets to do some important parts of his job.
Question: If we were to demote or let him go because of this, how should I document this to protect the company. We are trying to work with him to take meeting notes etc.

Send him to a Government authorized /constituted, medical board and take their opinion. Based on the report, you may terminate his service / or consider him for a different function /roll.